On the Thermal Energy Transfer between Free Electrons and Molecular Vibration

摘要

Thermally averaged probabilities for the electron dehyphen;excitation of the first vibrational level in N2are calculated from the discrete velocityhyphen;dependent collision cross sections measured by other workers. The results are presented for the electron temperature range 1000deg; to 5000deg;K. Over this range the probability is found to have an empirical temperature dependence of the formATmdash;1.8exp (Tfrac13;), withA=1.58times;10mdash;3. The probability increases from 10mdash;4at 1000deg;K to 10mdash;2at 5000deg;K. By comparison, the corresponding experimental probabilities for the dehyphen;excitation of N2vibration by nitrogen molecules are 10mdash;8at 1000deg;K and 3times;10mdash;4at 5000deg;K. These figures imply that in some circumstances an electron concentration of only 1 in 3times;106could halve the normal nitrogen molecular relaxation time at 1000deg;K, and that a concentration of 1 in 6times;103could do so at 5000deg;K. Existing spectrumhyphen;line reversal measurements of N2vibrational relaxation carried out in nozzlehyphen;expansion flows in the presence of small amounts of easily ionized Na and Cr atoms are reviewed in the light of these findings. It is shown that the electron concentrations in these experiments were too low to have significantly influenced the relaxation measurements.The relative rates of degradation of electron kinetic energy to the translational, rotational, and vibrational modes of N2are considered on the basis of the above calculations. From these considerations it is proposed that in many nonequilibrium molecular systems the kinetic electron temperature will be closely coupled to the vibrational temperature of the molecules. Some results of current expansionhyphen;flow experiments which support this proposed coupling process are briefly mentioned, and its possible importance in relation to the extent of chemical and ionization nonequilibrium in other expansion flows of shockhyphen;heated air in hypersonic nozzles and around rehyphen;entry vehicles is discussed. It is suggested that free electrons, although present perhaps in small quantities, could be important in energyhyphen;degradation processes in these systems. Other existing observations in N2plasms which appear to support the proposed coupling mechanism are reviewed.